A Unique Autocatalytic Process and Evidence for a Concerted-Stepwise Mechanism Transition in the Dissociative Electron-Transfer Reduction of Aryl Thiocyanates

Electrochemical reduction of p-methyl-, p-methoxy-, and 3,5-dinitrophenyl thiocyanates as well as p-methyl- and p-methoxyphenyl disulfides was investigated in acetonitrile at an inert electrode. This series of compounds reveals a striking change in the reductive cleavage mechanism of the S-CN bond in thiocyanates as a function of the substituent on the aryl ring of the aryl thiocyanate. With nitro substituents, a stepwise mechanism, with an anion radical as the intermediate, takes place. When electron-donating groups (methyl and methoxy) are present, voltammetric as well as convolution analyses provide clear evidence for a transition between the concerted and stepwise mechanisms based on the magnitude of the transfer coefficient alpha. Moreover, a very interesting autocatalytic process is involved during the electrochemical reduction of these compounds. This process involves a nucleophilic substitution reaction on the initial aryl thiocyanate by the electrochemically generated arenethiolate ion. As a result of this unusual process, the electrochemical characteristics (peak potential and peak width) of the investigated series are concentration dependent.